Figure 2.

Representative examples of independent and dependent (paired) two-group experimental design. Extracted ion chromatogram and box-plot/paired plot are shown for the features of interest. (A) A significantly down-regulated (p < 0.001) metabolite feature (m/z 171.005; METLIN MS/MS match, glycerol phosphate) in independent group design (control versus stressed bacterial population) was identified by using an independent parametric Welch t test. Welch’s t test is used to compare the means of two independent sample groups with the assumption that two-group variances may differ. (B) A significantly higher level (p < 0.001) of metabolite feature (m/z 309.279; METLIN hit, eicosenoic acid) in arterial blood plasma was determined by a paired nonparametric Wilcoxon test. Wilcoxon signed-rank test is a nonparametric alternative to the paired t test used to compare the related samples.